It has long been known that radio frequency (RF) signals may be coupled through an insulating material, such as glass, by mounting a conducting plate on each side of the insulating material, thereby forming a coupling capacitor. U.S. Pat. No. 1,715,952 to Rostron is one early reference teaching this general principle.
U.S. Pat. No. 2,829,367 to Rychlik applied this general principle to the problem of coupling a balanced line through an insulating window. Each conductor of the balanced transmission line is capacitively coupled by using a pair of conducting plates mounted on opposite sides of the glass. The patent teaches that such capacitive elements can be inserted in an electrical circuit with minimum loss if the point of insertion of the capacitive elements has a high impedance. The patent further discloses methods by which a low transmission line impedance can be converted into an effectively high impedance, for coupling through glass, and again restored to a low impedance by use of reciprocal transformers. The Rychlik system is unsuited for transmit operation and its performance is seriously degraded when the window is wet.
German Pat. No. 2,543,973 to Laurent describes a vehicle antenna, capacitively fed through a windshield, in which the antenna element is directly connected to, and supported by, the outside conducting plate.
Mobile Mark, Inc. offers an "OW-900" 800 megahertz windshield mounted antenna in which the center conductor of a coaxial feed line is connected to an inside coupling plate. A pair of parallel, spaced-apart quarter-wavelength vertical radiators are connected to the outside coupling plate. The shield conductor of the coaxial cable is connected to two "field cancelling" conductor strips which extend radially outward from the feed point on the inside surface of the windshield. The "field cancelling" conductors have no counterpart on the outside surface of the windshield.
Several problems are inherent in the design of the OW-900 antenna. One is that the antenna's radiation pattern is not omnidirectional, thereby causing the antenna to radiate poorly in some directions. Another problem is the radiation of substantially levels of RF energy into the passenger compartment of the vehicle during transmit operation. This is particularly important in the 800 megahertz and other VHF and UHF bands, where such radiation has been shown to have deleterious effects on human tissue. Lastly, the antenna elements used in the OW-900 system have virtually no vertical plane gain, resulting in a weaker transmitted and received signal than competing antenna systems.
Recently, it has been taught to provide an impedance matching circuit integrally with a windshield mounted, through-the-glass fed antenna system so as to lower the antenna's standing wave ratio. A low standing wave ratio is important for proper operation of radio transmitter units.
U.S. Pat. No. 4,089,817 to Kirkendall illustrates one such system in which a matching network is interposed between the center conductor of a coaxial feed line and an inside coupling plate. The shield conductor of the coaxial feed line is grounded to the vehicle body. The inside coupling plate comprises two irregularly shaped, rotably connected plates, thereby permitting the effective size of the inside plate, and consequently the value of the coupling capacitor, to be varied. This feature allows the matching circuit to be resonated by rotating one inside plate relative to the other. The Kirkendall antenna is mounted directly to, and is supported by, the outside coupling plate.
The Kirkendall system suffers from a number of drawbacks. One is the comparatively high level of stray radio frequency radiation inside the passenger compartment of the vehicle. Another drawback is the necessity to ground the shield conductor of the coaxial cable to the vehicle chassis. This connection must be made as close to the antenna as possible for optimum operation, thereby limiting the locations on the windishield at which the antenna can be mounted. Lastly, the Kirkendall coupling plates capacitively load the antenna, thereby rendering it less efficient than an unloaded antenna.
Another through-the-glass mobile antenna feed system with integral matching circuitry is shown in U.S. Pat. No. 4,238,799 to Parfitt. Parfitt discloses another system in which a matching network is interposed between the end of a coaxial feed line and an inside coupling plate. The ground conductor of the coaxial feed line is again connected to the vehicle chassis. The antenna is again mounted directly to, and is supported by, the outside conducting plate.
The Parfitt system, although believed to be illustrative of the state of the art in this technology, still present several important problems:
(a) A comparatively high level of stray radio frequency energy is again radiated into the passenger compartment of the vehicle during transmit operation.
(b) Parfitt's capacitive coupling plates again introduce a capacitive loading effects which renders the antenna less efficient than a comparable, unloaded antenna.
(c) The Parfitt system generally requires a grounding strap be connected from the inside matching circuit to the vehicle chassis for optimum operation. This, again, constrains placement of the antenna on the windshield, since the length of the grounding strap must be kept as short as possible.
(d) The Parfitt system is subject to marked variations in impedance and radiation characteristics when the windshield becomes wet, or when a foreign body, such as a windshield wiper, is moved in proximity to the base of the antenna.
Accordingly, a need remains from a through-the-glass antenna feed system that overcomes these drawbacks of the prior art.